Large gamma radiation instruments such as portal monitors and other large area detectors are used to examine truck trailers, automobile trunks, shipping containers, packages, luggage, personal effects, and similar items (hereinafter collectively referred to as “containers”) to determine whether any radiological material is located therein. These detectors also respond to natural background radiation originating from cosmic, airborne, and terrestrial sources. Under ideal circumstances the measure of radioactivity produced by any radiological material in a container being examined may be determined from the difference between (1) the radiation count rate seen by the detector(s) when the container occupies the survey space of the monitoring system, and (2) the radiation count rate observed when the monitoring system is “empty.”
Unfortunately, a complicating effect generally occurs when the container or its transporting vehicle being examined or both are so massive that they shield the detector(s) from viewing background radiation. In such circumstances the observed signal is the sum of detected radiation from any radiological material in the container or vehicle, and a reduced contribution from background radiation. A large vehicle has been shown to reduce the background radiation detected by substantial amounts, as much as ˜30%. Because the determination of radioactivity emanating from the container or vehicle is determined by a difference between the “occupied” and “empty” conditions, radioactivity emanating from a radiological source within a container or vehicle may not be reliably detected because the decrease in detected background radiation caused by the massive “sample” offsets the level of the detected emission from the radioactivity in the “sample,” resulting in an inaccurate reading. This situation is called “baseline depression” and has been shown to cause significant vulnerabilities in large radiation measuring systems such as portal monitors.
In attempts to overcome the effects of baseline depression in portal monitors, some systems have used measurements of radiation in different spectral “regions of interest.” The differences in such measurements have been calculated to compare the total number of gamma rays below an energy threshold versus the total number of gamma rays detected above that energy threshold (up to the maximum detectable energy limit of the detector): Eq'n 1 illustrates this for a gamma spectrum:
                              ROI          ⁢                                          ⁢          difference                =                                            ∑                              i                =                1                            n                        ⁢                                                  ⁢                          C              i                                -                      k            ·                                          ∑                                  i                  =                                      (                                          n                      +                      1                                        )                                                  maximum                            ⁢                                                          ⁢                              C                i                                                                        (                              Eq            ′                    ⁢          n          ⁢                                          ⁢          1                )            
In Eq'n 1, the Ci's are the counts in spectrum channels, n is a channel number selected to match the energy threshold, and “k” is a normalization constant that attempts to account for expected differences in background readings between the two regions of interest. An example of an application of this technique are calculations using the sum of all channel contents of the spectral region from 38-1,394 KeV (the region of man-made emitters) and the sum of the spectral regions from 1394-3,026 keV (the region containing mostly counts from naturally occurring gamma emitters). These spectral regions might be selected as regions of interest. The ROI difference of Eq'n 1 might then seem to provide a measure of man-made gamma radiation count. Unfortunately various factors such as detectable high-energy gamma rays, (e.g., 6.13 MeV gamma radiation emitted by 16N) affect a broad range of energies below the photopeak. Those energies change the spectral shape in a manner that adversely affects the calculation of the normalization constant used in Eq'n 1.
Various other techniques have been proposed for minimizing the effect of baseline depression, but they generally have unacceptable limitations. What are needed therefore are improved methods for reducing the effect of baseline suppression in portal and similar radiation monitoring systems where shielding effects may cloak the presence of radiological material.